Bentonite (smectite) clays are used widely in the construction of liners for hazardous waste landfills, slurry walls, industrial waste treatment lagoons, sewage lagoons, and tank forms. The utility of clays as waterproofing barriers or liners is derived from their ability to disaggregate upon hydration and form a dispersed phase of very small particles. These small clay particles effectively fill the void spaces between larger soil particles resulting in greatly reduced hydraulic conductivity. Thus, the primary function of clay liners, as well as synthetic geomembranes, is to impede the movement of water.
Smectite clays contain a net negative charge due to isomorphous substitution in the aluminosilicate layers. In nature, this charge is neutralized by inorganic cations such as Na.sup.+ or Ca.sup.2+ on the clay interlayers and external surfaces. Hydration of these cations in the presence of water initiates a separation of the clay layers causing a swelling of the clay. In smectites exchanged with monovalent cations having high hydration energies, e.g., Na.sup.+ or Li.sup.2+, the individual clay layers may become completely separated in the presence of water. However, the maximum distance between individual clay layers of divalent cation-exchanged smectites, e.g., Ca.sup.2+ or Mg.sup.2+, is about 19 .ANG.. Thus, in the construction of clay liners, Na-smectites are more effective in reducing hydraulic conductivity because they form small highly-dispersed particles in water.
The hydration of naturally occurring inorganic metal exchange ions on clays also imparts a hydrophilic nature to the clay layers. As a result, natural clays are ineffective in removing organic contaminants from water. However, by simple ion-exchange reactions, the naturally occurring inorganic exchange ions of clays can be replaced by a variety of organic cations and this may change the clay surface from hydrophilic to organophilic. These ion-exchange reactions can be used to form stable organo-clay complexes with high affinities for organic contaminants. Such organo-clays can be used in conjunction with conventional clays to increase the contaminant capabilities of clay barriers by immobilizing organic contaminants present in leachate. The sorptive properties of soils for organic contaminants also can be greatly enhanced by organic cation exchange of soil clays. Other possible environmental applications of organo-clays are in the stabilization, solidification of industrial wastes, and in water purification.
The sorptive properties of the smectite clays for organic contaminants are greatly modified by replacing inorganic cations (e.g., Na.sup.+, Ca.sup.2+) with quaternary ammonium cations (QACs) of the form [(CH.sub.3).sub.3 NR].sup.+. Organo-clays formed using small QACs, e.g., where R is a methyl or a phenyl group, exhibit pore structure which result in appreciable surface areas of .about.200 m.sup.2 /g. Recently, organo-clays, such as tetramethylammonium (TMA)- and trimethylphenylammonium (TMPA)-smectites, (Boyd, U.S. Pat. No. 5,268,109) have been characterized for their ability to remove various organic contaminants, such as benzene, alkylbenzenes, chlorinated phenols from water. Adsorption efficiencies were dependent on a number of factors. The size of the organic cation and the clay layer charge affected the surface area and pore structure of the organo-clays, and hence their adsorption efficiencies. Adsorption efficiencies also depended on the presence or absence of water. For instance, the adsorption of benzene, toluene and xylene vapors by the TMA-smectite was greater than their adsorption as solutes from water. Additionally, the adsorption of organic vapors by TMA-smectite was not strongly dependent on the size and shape of the adsorbate, whereas the extent of adsorption from water was significantly reduced as the size and shape of the adsorbate grew larger and bulkier. The lower adsorption efficiency of TMA-smectite in presence of bulk water was described as a water-induced sieving effect and attributed to shrinkage of the interlayer pore sizes by hydration of the TMA cations and/or aluminosilicate surfaces of the clay layers. Subsequent studies of the adsorption of organic contaminants from water by TMPA-smectites showed that the aluminosilicate surfaces are organophilic in nature and are the active adsorptive sites. Furthermore, unlike TMA-smectite, adsorption efficiencies of TMPA-smectite from bulk water were not strongly affected by adsorbate size and shape.
In accordance with the present invention, a new class of organo-clays, organo-phosphonium-smectites (e.g., tetramethylphosphonium (TMP)- and trimethylphenylphoshonium (TMPP)-clays) have been found for the adsorption of organic contaminants in the form of both vapors and solutes dissolved in water. Unexpectedly, it has been found that ion-exchangeable clays, such as montmorillonites or smectites, particularly the bentonites that have Na.sup.+, K.sup.+, Li.sup.+, Mg.sup.2+ or Ca.sup.2+ as their predominant exchangeable cations; as well as hectorite; saponite; nontronite; attapulgite; illite; zeolites; vermiculite, and the like, that are ion-exchanged with QPC ions comprised of four organic carbon moieties bonded to the central phosphorus atom including short chain alkyl moieties (C.sub.1 -C.sub.4); long chain alkyl moieties (C.sub.4 -C.sub.20), substituted or unsubstituted cycloalkyl; substituted or unsubstituted aryl or alkaryl moieties, e.g., benzyl or phenyl; alkenes, e.g., propene; aldehydes, e.g., acetaldehyde; ketones, e.g., acetophenone; alcohols, e.g., isobutylalcohol; carboxylic acids, e.g., valeric acid; esters, e.g., methylacetate, and combinations thereof, effectively adsorb or otherwise remove aliphatic and aromatic organic contaminants (e.g., hydrocarbons, chlorinated hydrocarbons, phenols etc.,) from gases, e.g., air, and from water.